Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a ceramic solid oxide electrolyte known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O2 molecule is split and reduced to two O−2 anions catalytically by the cathode. The oxygen anions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO2 at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.
A single cell generates a relatively small voltage and wattage, typically about 0.5 volt to about 1.0 volt, depending upon load, and less than about 2 watts per cm2 of cell surface. In practice, a plurality of cell modules are stacked and put into electrical series to meet volume and power requirements. The modules are separated by perimeter spacers whose thickness is selected to permit flow of gas to the anodes and cathodes as required and which are sealed axially to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent modules are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load. Typically, most or all of the planar elements in a stack are sealed to adjacent elements along their mating faces by seals which are formed of glass or other brittle materials. These materials can form excellent seals, especially at the high operating temperatures required for an SOFC; however, the seals must be sintered during assembly to cause the materials to be compressed and to flow into micro-irregularities in the surfaces to be sealed.
During assembly of the fuel stacks, a compressive load must be maintained during sintering of the stack assembly seals. This compressive load must then be maintained after the sintering process and during mounting of the assembly to the manifold at room temperature to ensure the integrity of the glass seals between the stack's components. During use of the assembled fuel cell system, components of the stack can change dimensions due to thermal expansion, which can place unacceptable stresses on the stack seals and which can cause mismatches in the heights of adjacent stacks.
What is needed is a means for providing a permanent and resilient compressive load to the stack during and after assembly to ensure the integrity of the glass seals between stack components at all times in the stack life.